1. Field of the Invention
The present invention relates to a scanning line interpolation device. More particularly, the invention relates to a scanning line interpolation device for converting an interlace signal into a non-interlace signal in image signal processing in a non-interlaced scanning CRT, a video printer, a matrix-type display device such as a plasma display, a liquid crystal display, an LED display, a field emission display, and a digital micromirror device, and the like.
2. Description of the Background Art
FIG. 15 shows a conventional scanning line interpolation device disclosed in Japanese Patent Application Laid-Open No. P05-68240A (1993). In FIG. 15, the reference numeral 101 designates a delay circuit for delaying input image data by a predetermined time interval; 102 designates a correlation judgement portion for judging the correlation between images; 103 designates a threshold value calculation circuit; 104 designates a binarization circuit; 105 designates an interpolation direction judgement circuit; 106 designates an interpolation calculation portion; 107 designates a selection circuit; 108 designates an adder; 109 designates a multiplier; the reference character Pi designates input image data; a, c, e, f, h and j designate reference pixel data extracted by the delay circuit 101 and required for interpolation; SH designates a threshold value; l, m, n, x, y and z designate results of binarization of the extracted pixels; IS designates interpolation direction selection data; PU and PD designate data selected for the interpolation; and Po designates an interpolation result.
The operation of the device shown in FIG. 15 will be described below. The two-dimensional image data Pi quantized using a predetermined sampling frequency is inputted to the delay circuit 101 which in turn extracts the reference pixels a, c, e, f, h and j. The reference pixels extracted by the delay circuit 101 are inputted to the threshold value calculation circuit 103, the binarization circuit 104, and the selection circuit 107. The threshold value calculation circuit 103 calculates the threshold value data SH to output the threshold value data SH to the binarization circuit 104. The binarization circuit 104 compares each of the reference pixels with the threshold value SH. If each of the reference pixels is not less than the threshold value, the binarization circuit 104 outputs "1." If each of the reference pixels is less than the threshold value, the binarization circuit 104 outputs "0." The binary data l, m, n, x, y and z corresponding respectively to the reference pixels a, c, e, f, h and j are inputted to the interpolation direction judgement circuit 105 for use as the addresses of an interpolation table in the interpolation direction judgement circuit 105. Then, the interpolation direction selection data IS is outputted from the interpolation direction judgement circuit 105. The selection circuit 107 selects one of vertical interpolation, right slant interpolation, and left slant interpolation, depending on the interpolation direction selection data IS. For the vertical interpolation, the selection circuit 107 selects the reference pixels c and h, and the adder 108 adds the reference pixels c and h together. Then, the multiplier 109 multiplies the sum of the reference pixels c and h by 1/2 to output the interpolation result Po. The selection circuit 107 selects the reference pixels e and f for the right slant interpolation, and selects the reference pixels a and j for the left slant interpolation.
For the interpolation of an uppermost or lowermost scanning line, the reference pixels in a single reference line are directly used for the interpolation or the reference pixels in the reference line which should be present on opposite side from the single reference line are set to "0".
FIG. 16 shows the relationship between an intended pixel Po located in an interpolation pixel position and the reference pixels. The circles of FIG. 16 denote the reference pixels, and the crosses denote interpolation pixels.
The interpolation table illustrated in FIG. 17 shows the relationship between the interpolation directions and the binary data l, m, n, x, y and z. The open circles ".largecircle." of FIG. 17 represent binary data "0"; the solid circles ".cndot." represent binary data "1"; the vertical lines ".vertline." represent the vertical interpolation; the slashes "/" represent the right slant interpolation; and the backslashes ".backslash." represent the left slant interpolation.
The conventional scanning line interpolation device constructed as above described always performs a function as a vertical low pass filter upon the input image, thus deteriorating high-frequency components and providing only an interpolation image with fuzzy horizontal edges. Additionally, as a result of the interpolation of a comer part at which horizontal and vertical edges intersect, the comer part is rounded off since the slant interpolation is selected in accordance with the interpolation table. In particular, the interpolation performed on image data with sharp edges such as the image data of characters and graphics results in conspicuous edge fuzziness and significantly reduced interpolation image qualities.
FIG. 18 shows a result of the interpolation performed by the conventional scanning line interpolation device. For the interpolation of horizontal edges, the conventional scanning line interpolation device always selects the vertical interpolation to provide median values as the interpolation values. For the interpolation of comer parts, the conventional scanning line interpolation device always selects the slant interpolation to provide white data as the interpolation values. The values in the interpolation line after the interpolation completely differ from those before the interpolation. In visual terms, the resultant image has fuzzy horizontal edges and rounded corners.
For example, image data "T" (ID1) as shown in FIG. 19A is contemplated which is comprised of first- and second-field image data ID2 and ID3 shown in FIGS. 19B and 19C, respectively. With such a configuration, when in-field interpolation is performed upon the first-field image data, interpolated image data ID4 has a joint part of "T" which is rounded off by means of an interpolation table t1 and a stem end to which white data is provided by means of an interpolation table t2, as shown in FIG. 19B. Likewise, the in-field interpolation performed upon the second-field image data designated by ID3 of FIG. 19C provides interpolated image data ID5.